Frame a basement wall with a decay-resistant or preservative-treated bottom plate (or a moisture barrier under it), studs at the spacing in 2021 IRC Table R602.3(5), fireblocking under Section R302.11, and a 1/2-inch gypsum thermal barrier over any foam plastic.
A basement wall here means a wood stud wall built on a concrete slab, either furred out from the foundation wall or standing free as a partition. The 2021 International Residential Code (IRC) frames it like any interior wall and adds decay rules where wood meets concrete. The code edition your jurisdiction has adopted, plus any local amendments, governs, and foam, sealants and fasteners follow their manufacturers' instructions.
Key facts
- Under 2021 IRC Section R317.1 (item 3), sills and sleepers on a slab in direct contact with the ground must be naturally durable or preservative-treated wood unless an impervious moisture barrier separates them from the slab.
- Under 2021 IRC Section R317.1 (item 7), wood furring or framing attached directly to the interior of a concrete wall below grade needs the same protection unless an approved vapor retarder is applied between the wall and the wood.
- Under 2021 IRC Section R302.11, stud wall cavities, including furred spaces, are fireblocked vertically at the ceiling and floor levels and horizontally at intervals not exceeding 10 feet.
- Under 2021 IRC Section R602.5, an interior nonbearing wall may use 2x3 studs at 24 inches on center, or 2x4 flat studs at 16 inches on center where it is not part of a braced wall line.
- Under 2021 IRC Section R316.4, foam plastic is separated from the interior by at least 1/2-inch gypsum wallboard, 23/32-inch wood structural panel, or a material tested to NFPA 275.
- Under 2021 IRC Section R305.1, habitable basement space needs a ceiling height of at least 7 feet, and beams, girders and ducts may project to within 6 feet 4 inches of the finished floor.
How do you frame a basement wall?
Work through the wall in this order, so the decisions that affect lumber are made before anything is cut.
- Decide whether the wall is bearing. A bearing partition is built, framed and fireblocked like an exterior wall (R602.4). A wall that carries only itself falls under R602.5. Joist direction and what sits above decide it, as the load-bearing walls guide explains.
- Check the finished ceiling height against Section R305.1.
- Protect the wood from the concrete with a treated plate or moisture barrier.
- Cut the plates and studs. Studs have full bearing on a nominal 2-by plate at least as wide as the studs (R602.3.4). A bearing wall takes a double top plate with end joints offset not less than 24 inches (R602.3.2). Treat field-cut ends and holes in treated wood on site (R317.1.1).
- Lay out and nail the studs. Studs are continuous from sole plate to top plate (R602.3).
- Fix the plate to the slab and the top plate to the framing above.
- Fireblock, then insulate and cover.
Names for every part, and how openings are framed, are in wall framing basics.
What should the bottom plate sit on, and what should it be made of?
The bottom plate should be naturally durable or preservative-treated wood, or sit on an impervious moisture barrier. That follows 2021 IRC Section R317.1, which requires decay protection with naturally durable wood or wood treated in accordance with AWPA U1 in these basement-relevant cases:
- Item 3: sills and sleepers on a concrete or masonry slab in direct contact with the ground, unless separated from the slab by an impervious moisture barrier.
- Item 7: wood furring or other wood framing attached directly to the interior of exterior masonry or concrete walls below grade, except where an approved vapor retarder is applied between the wall and the wood.
- Item 9: wood columns in contact with basement floor slabs, unless supported by concrete piers or metal pedestals projecting not less than 1 inch above the floor and separated from the pier by an impervious moisture barrier.
Fasteners and connectors in contact with preservative-treated wood must also suit it, under Section R317.3. The ground contact lumber guide lists all nine R317.1 locations and the tag information to look for.
The sections used here do not give a fastener type or spacing for tying an interior bottom plate to a basement slab. The anchor bolt rules in Section R403.1.6 cover wood sill plates on continuous foundations and sole plates on monolithic slabs, which is a different condition. Use the fastener manufacturer's instructions and ask your building official what they require.
What stud spacing does a basement wall need?
A basement wall needs the stud size and spacing that match what it carries. The table below shows which rule applies, and the section to read for the numbers.
Framing rules by basement wall type, 2021 IRC Sections R602.3.2, R602.4 and R602.5. A summary of code text, not a span or load table.
| Wall type | Stud rule | Top plate | Section |
|---|---|---|---|
| Interior bearing partition | Same as an exterior wall: Table R602.3(5) | Double, end joints offset at least 24 inches | R602.4, R602.3.2 |
| Interior nonbearing partition | 2x3 at 24 inches on center, or 2x4 flat at 16 inches on center if not part of a braced wall line | At least a single plate | R602.5 |
| Furred wall carrying only its own weight | Treated as an interior nonbearing wall, with the R317.1 item 7 decay rule against concrete | At least a single plate | R602.5, R317.1 |
For bearing walls, Table R602.3(5) gives a 2x4 bearing stud 24 inches on center when it carries a roof-ceiling assembly only or one floor only, and 16 inches on center when it carries one floor plus a roof-ceiling assembly. A 2x6 bearing stud may be 24 inches on center for those cases. Listed bearing heights are 10 feet between points of lateral support, and bearing walls must be sheathed on at least one side or have bridging not more than 4 feet apart measured vertically from either end of the stud. The full table, with the exceptions in Section R602.3.1, is in the stud spacing guide.
How do you nail a basement stud wall?
Fastening for a basement partition, from 2021 IRC Table R602.3(1). The table lists other fastener options for each item.
| Item | Connection | Fastener | Spacing or location |
|---|---|---|---|
| 17 | Top or bottom plate to stud | 3-16d box (3-1/2 in. x 0.135 in.) or 2-16d common (3-1/2 in. x 0.162 in.) | End nail |
| 17 | Top or bottom plate to stud | 4-8d box (2-1/2 in. x 0.113 in.) or 4-10d box (3 in. x 0.128 in.) | Toe nail |
| 13 | Top plate to top plate | 16d common (3-1/2 in. x 0.162 in.) | 16 inches on center, face nail |
| 14 | Double top plate splice | 8-16d common (3-1/2 in. x 0.162 in.) | Each side of the end joint, minimum 24-inch lap length each side |
How low can a finished basement ceiling be?
A finished basement room that counts as habitable space needs a ceiling height of at least 7 feet under 2021 IRC Section R305.1. Bathrooms, toilet rooms and laundry rooms need at least 6 feet 8 inches.
Beams, girders, ducts and other obstructions in a basement with habitable space may project to within 6 feet 4 inches of the finished floor (Exception 3). Beams and girders spaced at least 36 inches apart in clear finished width may project to no more than 78 inches from the finished floor (Exception 4). Basement areas with no habitable space need at least 6 feet 8 inches (R305.1.1). See minimum ceiling height for the full section, and built-up beam for a beam overhead.
Where does a basement wall need fireblocking?
Fireblocking goes in the concealed spaces of stud walls and partitions, including furred spaces and parallel or staggered rows of studs. Section R302.11 sets it vertically at the ceiling and floor levels and horizontally at intervals not exceeding 10 feet. It is also required at interconnections between concealed vertical and horizontal spaces, such as soffits and drop ceilings, and around vents, pipes, ducts, cables and wires at ceiling and floor level. Section R602.8 points back to the same rules.
Permitted materials under R302.11.1 include two-inch nominal lumber, 1/2-inch gypsum board, and batts or blankets of mineral wool or glass fiber securely retained in place. Unfaced fiberglass batts must fill the entire cross section of the cavity to a height of at least 16 inches. The fire blocking guide covers the materials and the draftstopping difference.
Do you need a thermal barrier over basement foam insulation?
Yes, in most cases. Foam plastic must be separated from the building interior by a thermal barrier of at least 1/2-inch gypsum wallboard, 23/32-inch wood structural panel, or a material that meets both NFPA 275 fire tests (R316.4). Exceptions are listed in Section R316.5. Examples:
- R316.5.1: no thermal barrier where the foam sits in a masonry or concrete wall and is separated from the interior by at least 1 inch of masonry or concrete.
- R316.5.11: spray foam on sill plates and headers, or in the perimeter joist space, may skip the barrier if it is not thicker than 3-1/4 inches, has a density of 0.5 to 2.0 pounds per cubic foot, and has a flame spread index of 25 or less and a smoke-developed index of 450 or less.
How much insulation the wall needs is set by the energy code, which is outside the sections used for this article.
What this article does not cover
Wall bracing under Section R602.10, waterproofing and damp-proofing, egress openings for basement bedrooms (see egress window requirements), electrical and plumbing, and insulation values are separate checks. The tables and sections quoted are prescriptive limits for their stated conditions only.
Summary
A basement stud wall needs a treated or decay-resistant bottom plate, or a moisture barrier under it, wherever it meets a slab or below-grade concrete (R317.1). Stud spacing follows Table R602.3(5) for bearing walls and Section R602.5 for nonbearing walls. Fireblock the cavities under R302.11, and cover any foam plastic with a thermal barrier under R316.4. Confirm local amendments with your building official.
Sketch the framing above the wall
The basement wall carries or braces the floor framing over it. Framing Plan is a preliminary drawing aid: it draws joist layouts, beams and openings and exports PNG, PDF and DXF. Its sample span profile uses demo figures and the editor works in millimetres, so it does not check code or size members. Open the framing studio to lay out the joists over your wall lines, then have the layout verified for your project.